1. Field of the Invention
This invention relates to a magnetic transducer head and more particularly to such a head formed of composite magnetic material, viz. the ferromagnetic oxide material and the ferromagnetic metal material.
2. Description of the Invention
With increase in recording density on magnetic tapes used as recording media for video tape recorders (VTRs), magnetic tapes having a high residual flux density Br and a high coercive force Hc, for example metal magnetic tape in which metal magnetic powder is coated on a non-magnetic substrate with a binder to form a magnetic recording layer, are being used in increasing numbers. When the magnetic transducer head is to be used with the metal tape, the magnetic field strength of the magnetic gap of the head must be elevated in order to cope with the high coercive force of the tape. It is also necessary to reduce the track width of the magnetic transducer head with increase in recording density. There are known various magnetic transducer heads designed to meet these demands, such as the magnetic transducer head with the narrow track width shown in FIG. 1. The major portion of the magnetic transducer head shown in FIG. 1 is formed of glass or the like non-magnetic materials 1A, 1B, and a ferromagnetic metal thin film 2 having a thickness equal to the track width is sandwitched between these non-magnetic materials centrally of the magnetic head. This film 2 is prepared by forming a high permeability alloy such as Sendust (fe--Al--Si alloys) on the non-magnetic material 1A in the form of a core half by physical vapor deposition, such as sputtering. While the track width can be reduced in this manner, the path of magnetic flux is defined only by the metal thin film 2 and hence the operational efficiency is lowered by reason of increased magnetic reluctance. The metal thin film 2 needs to be formed to a film thickness equal to the track width by the physical vapor deposition such as sputtering. Hence the preparation of the magnetic head is considerably time-consuming in view of the low deposition rate achievable with the physical vapor deposition. Since the film 2 needs to be formed on a large area, the number of items that can be dealt with by a sputtering unit is necessarily limited so that the heads cannot be mass-produced efficiently. The metal films 2 of extremely small film thickness are placed in contact with each other for formation of the magnetic gap of the magnetic transducer head, with the result that accuracy in the gap size and hence the operational reliability are lowered.
The magnetic transducer head shown in FIG. 2 is prepared in such a manner that, for increasing the magnetic field strength of the magnetic gap, ferromagnetic metal thin films 4 such as Sendust are formed on the magnetic gap forming surfaces of the core halves of ferromagnetic oxides by using a physical vapor deposition, such as sputtering, and the core halves are bonded together by the glass 5. Although the magnetic reluctance of the magnetic transducer head of FIG. 2 formed of the composite magnetic material can be made lower than in the case of the transducer head shown in FIG. 1, the films 4 are formed in a direction normal to the path of magnetic flux so that the playback output is lowered because of eddy current loss. Additional gaps may also be formed between the ferromagnetic oxide cores 3 and metal magnetic films 4 thus detracting from the operational reliability of the transducer head.
Also known is a magnetic transducer head formed of composite magnetic material and having its magnetic gap forming surface inclined with respect to its surface forming the ferromagnetic metal film. For example, FIG. 3 shows in plan view the contact surface with the magnetic tape of the magnetic transducer head described in a Japanese Patent Kokai No. 155513/1983.
The magnetic transducer head shown in FIG. 3 is comprized of core halves or core elements 150, 151 formed of ferromagnetic oxides, such as Mn--Zn ferrite. Ferromagnetic metal thin films 155, 156 such as Sendust are deposited on both sides of and astride ferrite portions 153, 154 projecting towards a surface forming the magnetic gap 152. The numeral 157 designates a reinforcing glass material. The magnetic gap of the head is formed by the thin films 155, 156 of a ferromagnetic metal material deposited in the neighborhood of the tip ends of the projecting ferrite portions 153, 154. With these films 155, 156 of the ferromagnetic metal material, the growth direction of the columnar grain structure at the tip ends of the projecting ferrite portions 153, 154 is different from that at both inclined sides thereof, in such a manner that the crystals grow on both sides in parallel and uniformly with a constant angle relative to said sides whereas the crystals growths at the tip ends are in a fan shape that is, the crystals are spread apart towards their distal ends. The result is that magnetic permeability of the ferromagnetic thin films 155, 156 formed on the tip ends is lowered with resultingly lowered recording characteristics and playback output of the magnetic head.
It would be worthwhile to consider here as to know the surface conditions of e.g. the ferrite substrate surface affect the film forming process when the ferromagnetic metal thin film is formed by physical vapor deposition on the ferrite substrate.
In general, a thin magnetic film to be formed by a physical vapor deposition process is affected in known manner by the under-layer conditions. Besides the crystal structure of the substrate and of the under-layer film formed as an extremely thin under-layer on the substrate, also noteworthy are the geometrical configuration and uniformity of the substrate surface.
FIG. 4A is a photograph taken with a scanning electron-microscope (SEM) of a two-layered Sendust film formed by sputtering on the ferrite substrate with a SiO.sub.2 film 500 .ANG. thick between the Sendust layers. This figure shows, along with another SEM photograph of FIG. 5A, the effect of the ferrite substrate surface configurations on the film formation. FIGS. 4B and 5B are sketches showing only the main features appearing in the photographs of FIGS. 4A and 5A, respectively.
FIG. 4A shows the Sendust film formed on a planar ferrite substrate surface. As seen from this photograph, Sendust film surfaces 159A, 159B formed on the planar surface are uniform and the growth of the columnar grain structure of the crystals appearing in sections 160A, 160B of the Sendust film is uniform and extends parallel to the thickness of the film. In this photograph, the broken section is taken not only of the Sendust film but of the ferrite substrate and the broken section is viewed with the scanning electron-microscope from an oblique direction. On the section of the ferrite substrate 161 is seen the section 160A of the first Sendust layer followed by the section 160B of the second Sendust layer. The film surfaces 159A, 159B belong to the first and second Sendust layers, respectively. The thin lines appearing on the surface 159B of the second Sendust layer represent micro line imperfections on the polished surface of the ferrite slice propagated to the Sendust film and do not affect the magnetic permeability of the film. The photograph is shown in a topsy-turvy state, that is, with the upper side down and vice versa.
FIG. 5A shows the Sendust film formed on an irregular surface of the ferrite substrate. The photograph shows the irregular surface 162 of the Sendust film corresponging to the original irregular surface ferrite substrate. This is indicative of the competitive growth of the crystal grains that is not observed when the crystals are allowed to grow on a smooth planar surface. Also the direction of the columnar crystal growths are not parallel, as may be seen in a section 163 of the Sendust film, but the columnar crystal growths are spread apart in a fan shape on the protuberant portions of the ferrite substrate. In the present SEM photograph, the broken section is taken not only of the Sendust film but of the ferrite substrate and the viewing direction is from the oblique upper side. A Sendust film section 163 is seen above a ferrite substrate section 164. A boundary line 164A between the sections 163, 164 represents a protuberant portion on the ferrite substrate surface.
The Sendust film formed on the ferrite substrate having recesses and protuberances present the direction of columnar crystal growths different with the inclination of the recesses. Thus the direction and size of the columnar crystals are different with the profile and inclination of the bottom of the substrate recess. The Sendust film surface 162 is also disturbed and the crystal structure of the film differs markedly with different inclination on the bottom of the recess. Such differnce in the crystal grain structure accounts for a great difference in the magnetic permeability of the Sendust film. The photograph in this figure is again taken in a topsy-turvy position.
It should be noted that, because magnetic permeability as well as anisotropic properties (the direction of easy magnetization) of a ferromagnetic film depends notably on the film structure, it is desirable that the magnetic film that makes up a magnetic transducer head, especially one used for magnetic recording and reproduction, be uniform in structure. For instance, it is required that columnar crystals of the aforementioned Sendust film should grow uniformly and in one direction. Should the orientation of crystal growth not be uniform in a magnetic film, a certain portion of the film exhibits proper magnetic properties while the remaining portion thereof exhibits inferior magnetic properties (effect of anisotropy).
In FIG. 6, there is schematically shown the structure of a Sendust film, that is, the orientation of the columnar crystal growths, when the Sendust film is deposited as by sputtering on and astride the projecting portion of the ferrite substrate shown in FIG. 3. It is seen from FIG. 6 that columnar crystals of the Sendust film 171 grow uniformly and parallel to each other on both sides 170A of the projecting portion 170 but are spread apart from each other towards the distal ends at a tip end 170B. When the Sendust film 171 deposited on the tip end 170B is ground for forming a magnetic gap surface 172, the film structure at or near the gap surface 172 is different from that on the sides 170A. Thus, with the magnetic transducer head of the composite magnetic material making use of the Sendust film 171 deposited on the projecting portion 170, when the Sendust film 171 on the sides 170A exhibits higher magnetic permeability in the direction of the path of magnetic flux, the film 171 near the tip end 170B exhibits only poor magnetic permeability.
Instead of depositing, for instance, a Sendust film on and astride the projecting portion of the ferrite substrate, it is also feasible to deposite the Sendust film 177 only on one side of the projecting portion 175, as by sputtering, with a masking plate 176 placed to cover the other side of the projecting portion 175. However, the masking plate 176 gives rise to a shadowing effect because a plate thickness in excess of several tens of microns is required in consideration of handling and mask-alignment and by reason of molding constraints. As a result of the shadowing effect, the film structure of the Sendust film 177 formed in the vicinity of the tip end 175B of the projecting portion 175B and hence magnetic permeability characteristics are different from those of the film structure on the side 175A. Thus, when the Sendust film 177 deposited on the tip end 175B is ground to a magnetic gap surface 178 of the magnetic transducer head, it is not possible with this magnetic head to provide both the film portion on the tip end 175B and the film portion on the side 175A with high magnetic permeability along the path of magnetic flux.
It is also feasible to get the gap surface ground further so as to render the film structure at the tip end 175B of the Sendust film identical with that at the side 175A. However, in this case, the ferrite portion is exposed on the magnetic gap surface 179 of the magnetic head, with the resulting inconvenience that a sufficient magnetic recording cannot be obtained on the track portions of the high coercive force magnetic tape, such as metal tape, corresponding to the width of the exposed ferrite portion.
FIGS. 11 and 12 show in plan views two further examples of the contact surface with the tape of the prior-art magnetic heads, with the magnetic gap portion being shown to an enlarged scale. With the magnetic head shown in FIG. 11, the Sendust films 183 for example are provided only on both sides of the ferrite portions projecting towards the planar surface 180 forming the gap and the ferrite portion is exposed on the planar surface 180 forming the gap. The numeral 184 designates a reinforcing glass packing material. This magnetic transducer head makes use of the Sendust film 183 formed on the planar surface and hence does not suffer from the above described non-uniform film structure. However, the magnetic recording on a high coercive force magnetic tape is insufficient by a width of the ferrite portion exposed on the magnetic gap surface, and the magnetic recording characteristics and playback output is correspondingly lowered.
In the magnetic transducer head shown in FIG. 12, a Sendust film 187, for example, is formed on ferrite portions and non-magnetic glass having high melting point portions 188 of core elements 185, 186, so that the head is formed of composite magnetic material, viz. ferrite and Sendust. The numeral 190 designates a glass 190 having melting point lower than that of glass 188. The magnetic gap 189 of the magnetic transducer head is formed by the portions cf the Sendust film 187A running parallel to the path of magnetic flux so that the Sendust film 187A in the vicinity of the magnetic gap 189 is of a uniform film structure. However, the Sendust film portion, 187B corresponding to the bend or knee of the Sendust film 187 and thus extending over two planar surfaces is not of uniform film structure, so that Sendust film 187 as a whole is not constant in magnetic permeability. Also, in this magnetic transducer head, the Sendust film portion 187A needs to be of a film thickness corresponding to the track width. Because of the slow deposition rate of the film possible with the physical vapor deposition, the process of fabrication of the magnetic transducer head is time-consuming.
The Japanese Patent Kokai No. 169214/1981 shows a magnetic transducer head in which, as shown in FIG. 13, junction surfaces 195, 196 of magnetic alloy films 191, 192 and ferrite portions 193, 194 are at an acute angle with respect to the confronting surfaces of the head gap 197 or to a direction normal to the relative running direction of the magnetic recording medium. However, with the magnetic transducer head shown in FIG. 13, the magnetic alloy films 191, 192 are mounted in opposition to each other in other portions than the head gap 197 so that a crosstalk may be caused especially in the longer wavelength signals by picking up the signals of neighboring tracks or the signals of every other track and a means for avoiding this effectively has not been found to date. In addition, local wear may be caused by the head gap 197 offset to one side edge of the head chip. The magnetic alloy films 191, 192 abut on each other in such a manner that the direction of columnar crystal growths of the film 191 does not coincide with that of the film 192 and uniform magnetic properties are difficult to achieve with the head gap 197.
Although the crystalline Sendust film has been given hereinabove as an example of a thin ferromagnetic film, a uniform film structure is also required when an amorphous alloy is used for forming the thin film. Since the film is amorphous, it is not the uniformity in the crystal grain structure but the uniformity in magnetic anisotropy that matters. If the amorphous alloy is deposited on a planar surface for forming a thin film, magnetic anisotropy is identical throughout the film. However, when the alloy is deposited astride a projecting portion and a planar portion, the magnetic domain structure or the magnetic permeability is not uniform.